1. Field of the Invention
The present invention relates to sensors or detectors for use in cores of nuclear reactors.
2. Prior Art
Radiation detectors which do not require external power but merely use an emitter, a collector, and an insulator material between the two to generate an electric current that is indicative of the intensity of the radiation are well known. For example, an article in "Atomnaya Enegriya", Vol. 10, No. 1, Pages 72-73, January, 1961 (a Russian publication) discloses a detector using an emitter, a collector and a dielectric material between the two to provide for measurements of neutron flux values.
In addition, a similar type of device is shown in U.S. Pat. No. 3,375,370 issued Mar. 26, 1968 to Hilborn.
Thus, while the principles of operation of such radiation detectors are well known, the problems of manufacturing reliable detectors has persisted. These detectors generally are of small diametral size, but of substantial length. For example, the detectors may be in the range of 1/16 inch in diameter, but may range from 30 feet to 130 feet or so in length.
At the present time, rhodium emitters insulated from an outer metallic conductor which is substantially more transparent than rhodium to neutron flux have been used quite extensively but satisfactory designs suitable for use in a nuclear reactor have not been completely successful. The insulation material generally used in the prior art is a ceramic insulation, and specifically highly compacted aluminum oxide or magnesium oxide insulators have been used extensively. Generally, a type of cable having an outer tubular jacket, and internal ceramic insulators surrounding a lead wire is assembled and then mechanically swaged. The swaging results in a density of insulation of typically 8% of a solid material and also causes elongation and other deformation of the inner wire. This same assembly technique has been used for the rhodium sensor. The sensor assembly is then brazed to the cable, with the sensor being brazed to the center lead wire, and the two outer sheaths being brazed together. This process places two brazed joints in an area where radiation is high and such joints have resulted in many failures. Once the joint between the two outer sheath sections fails, moisture destroys the insulating capabilities of the insulating material and the sensor is no longer useful. Also, the brazed joint between the lead wire, which is typically a nickel alloy such as Inconel, and the rhodium element itself will fail.
Insulation materials, such as magnesium oxide and aluminum oxide, absorb significant amounts of the charged particles emitted by the rhodium emitter so that a high density of insulant reduces the signal level. Further, particularly in the case of magnesium oxide, it is almost impossible to sufficiently dry out a length of that insulation once it has become moist, either in the manufacturing process or through mishandling at a subsequent time.
The conventional way of assembling long lengths of magnesium oxide or aluminum oxide cable is to begin with relatively short (6 inch) cast cylindrical sections of the insulating material which are slid over a wire and then the wire and insulation cylinders are placed into a tube which is subsequently drawn through a die or swaging machine compacting the outer metal tube against the ceramic cylinder sections and then against the inner wire. This can result in nicks in the conducting wire between individual sections and in some cases actual breakage of the wire since the material must be crushed and compacted before flowing results. In addition location of the hole in the cast cylindrical sections may vary significantly from the center axis resulting in poor radial symmetry of the wire with respect to the tube. To fabricate the relatively long lengths required for in-core detectors without splices, it is customary to redraw or re-swage the assembly several times. This results in non-uniform deformation and weakening of the wire. Further it is almost impossible to cast the length of ceramic tubing completely uniform so that the resulting structure, after drawing, is a wire held within an outer tube which is not normally located in the center of the tube throughout its length but may snake back and forth along the length of the tube. Emitters which are not uniform in cross section or centering from one sensor to another will yield non-interchangeable outputs and cause difficulties in measurement in application.